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"I cannot express enough my gratitude to the entire NASA team for the support during my career and especially the last 14 months as your acting administrator," Lightfoot said in a "bittersweet" goodbye note sent to the agency staff on Monday.

"The grit and determination you all demonstrate every day in achieving our missions of discovery and exploration are simply awe inspiring.

"I leave NASA blessed with a career full of memories of stunning missions, cherished friendships, and an incredible hope for what is yet to come," Lightfoot said.

Lightfoot, further said that he plans to work with the White House "on a smooth transition to the new administrator".

His retiement adds more uncertainty about the agency's leadership.

The White House nominated Bridenstine to fill NASA's top post in September 2017 and he resubmitted the nomination in January.

In both cases his nomination has been advanced by the Senate Commerce Committee on a strict party-line vote, but Bridenstine lacks the 50 votes needed to be confirmed.